The present invention relates to a magnetic resonance (MR) data acquisition method, an MR image construction method, and a magnetic resonance imaging (MRI) system. More particularly, the present invention is concerned with an MR data acquisition method capable of acquiring data, which is used to construct a water component-enhanced/fat component-suppressed image or a fat component-enhanced/water component-suppressed image, with a repetition time TR set to a desired value, an MR image construction method for constructing the water component-enhanced/fat component-suppressed image or fat component-enhanced/water component-suppressed image using the data acquired according to the MR data acquisition method, and an MRI system in which the methods are preferably implemented.
In the steady-state free precession (SSFP) method in which an NMR signal is induced by the steady-state free precession of the transverse magnetization of spins, the fast imaging employed steady-state acquisition (FIESTA) method, or the fast imaging with steady-state free precession (True FISP) method, the fat saturation RF pulse method, the fluctuation equilibrium MR (FEMR) method, or the linear combination SSFP (LCSSFP) method is used in combination in order to construct a water component-enhanced image or a fat component-enhanced image.
According to the LCSSFP method, a repetition time TR is set to an out-of-phase time T_out during which spins in water and spins in fat are out of phase with each other due to chemical shifts. Data D—φ1 is acquired according to a steady-state pulse sequence (FIG. 29) stipulating that when φ1=3π/2 is established, the phase of an RF pulse is varied in order of 0×φ1, 1×φ1, 2×φ1, 3×φ1, etc. Data D—φ2 is acquired according to a steady-state pulse sequence (FIG. 30) stipulating that when φ2=π/2 is established, the phase of an RF pulse is varied in order of 0×φ2, 1×φ2, 2×+2, 3×φ2, etc. Data processing expressed as D—φ1+exp(i×π/2)×D—φ2 is performed in order to produce data Dw. The data Dw is used to construct a water component-enhanced/fat component-suppressed image Gw. Moreover, data processing expressed as D—φ1−exp(i×π/2)×D—φ2 is performed in order to produce data Df. The data Df is used to construct a fat component-enhanced/water component-suppressed image Gf (refer to, for example, Non-patent Document 1 or Patent Document 1).
[Non-patent Document 1] “Linear Combination Steady-state Free Precession MRI” written by Vasanawala et al. (Magnetic Resonance in Medicine, Vol. 43, 2000, pp. 82-90)
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 2003-52667 ([0009] and [0010])
According to the LCSSFP method, the repetition time TR must be set to the out-of-phase time T_out. The out-of-phase time T_out depends on a magnetic field system. For example, when the magnetic field system offers a magnetic field strength of 0.2 T, the out-of-phase time is 20 ms. When the magnetic field strength is 0.35 T, the out-of-phase time is 10 ms. When the magnetic field strength is 0.7 T, the out-of-phase time is 5 ms. When the magnetic field strength is 1.5 T, the out-of-phase time is 2.3 ms.
However, for example, when the magnetic field system offers a magnetic field strength of 0.2 T, if the repetition time TR is set to 20 ms, it poses a problem in that a scan time gets long. On the other hand, for example, when the magnetic field system offers a magnetic field strength of 1.5 T, if the repetition time TR is set to 2.3 ms, hardware must incur a large load. Namely, the conventional LCSSFP method stipulates that the repetition time TR is set to the out-of-phase time T_out. The conventional LCSSFP method is therefore hardly implemented in a low-field strength system or a high-field strength system.